Needle δ 13 C values of different tree species. Plants with different photosynthetic pathways (C3 and C4) have different leaf δ13C values due to differences in primary carboxylases34. The δ13C values in C3 plants range from − 20‰ to -35‰, whereas those in C4 plants range from − 7‰ to -15‰35. P. armandi and P. yunnanensis are C3 plants (the δ13C values ranged from − 25.19‰ to -29.81‰ in this study) (Fig. 5). These values are consistent with those of Radermachera sinica, Sapium rotundifolium, Sterculia euosma, Schefflera octophylla, Alchornea trewioides, and Vitex negundo, which grow on continuous dolomite surfaces in subtropical China. These six C3 species had the same range of δ13C values36. The range of leaf δ13C values of C3 plants differs for different climatic zones37. Our study area has a subtropical monsoon climate with only 912.2 mm annual precipitation (Fig. 2), but P. yunnanensis and P. armandi needles have higher δ13C values than typical subtropical species (from − 31.1‰ to -30.5‰37, indicating that both species exhibit high WUE. Guo et al.38 found that different tree species had significantly different δ13C values (Table 1). The P. armandi needles had a higher δ13C value than P. yunnanensis, indicating that P. armandi exhibited higher WUE than P. yunnanensis. This difference was attributed to differences in the genetic traits of the species, resulting in different strategies for adapting to environmental change.
Leaf δ 13 C values of plants of different stand ages. The natural abundance of stable carbon isotopes in plant leaves changes during growth39. A comparative study of leaf δ13C values of42 grassland species in arid regions of the northwestern United States showed that the δ13C values of 1-year-old plants were lower than those of perennial plants40. Leaf δ13C analysis of several major warm-temperate deciduous tree and shrub species in Beijing's Dongling Mountains revealed a gradual decrease in the δ13C value from early to late growth41. In this study, stand age had a significant effect on the needle δ13C value (Table 1). The value was significantly higher in young stands than in old stands of P. armandi, especially in winter. The δ13C value was 4.30% higher in young stands than in middle-aged stands, indicating that a change occurred during growth. This result is consistent with the study by Zhang et al.42, who focused on different ages of Populus euphratica Oliv on the Tarim River. The WUE decreased with the increasing stand age. Kong et al.43 studied the WUE of different stand ages of Cunninghamia lanceolata (Lamb.) Hook plantations. The ranking of the WUE was current year > 1 year > 2 years > 3 years. The leaf δ13C values increased with the increasing stand age, which might be related to the extent of the root system. As the plant grows and matures, the root system expands, and its leaf δ13C decreases, resulting in low WUE. P. sylvestris showed similar characteristics, with older needles showing a gradual decline in water use44, whereas Casper et al.45 observed the opposite in Cryptantha flava. The WUE was significantly higher in mature stands than in younger stands, which was attributed to the fact that mature plants maintained a higher photosynthetic rate during the growing season, whereas stomatal conductance is similar to that of young plants.
This study showed no significant difference in the δ13C values of P. yunnanensis needles between young and middle-aged stands, suggesting that the δ13C values in P. yunnanensis needles are relatively stable during growth. This result is consistent with the findings of Kong43, who analyzed the factors influencing WUE in major forest types in the subtropics. The author stated that the photosynthetic rate of leaves was influenced by the resistance of the leaf pulp, limiting CO2 diffusion with increasing age46. Biochemical limitations exist due to changes in primary carboxylase activity and photochemical properties due to changes in X46. A decrease in the photosynthetic rate and stomatal conductance of needles may indicate low WUE.
δ 13 C values of plant leaves in different seasons. Water is the primary environmental factor limiting plant growth and development, and water deficiency or excess water can severely limit plant productivity. A survey of global conifer δ13C values showed that in the absence of a water deficit, the δ13C value reached an asymptotic value, precipitation and transpiration reached equilibrium, and stomatal conductance reached the maximum47. Under water stress conditions, the productivity of plants decreases. A study of Quercus prinus found that the leaf δ13C value was lower in autumn before the leaf drop than in the middle of summer48. Seasonal variations were observed in the δ13C value of four major shrub species (Haloxylon ammodendron, Nitraria tangutorum Bobr, Calligonum mongolicum, and Tamarix ramosissima). The relationships between the δ13C values and meteorological factors in an arid desert region were analyzed, and significant differences in the δ13C value were observed between different months and species38.
In this study, the needle δ13C values in young stands of P. armandi were significantly higher in summer, autumn, and winter than in spring, whereas there was no significant difference in middle-aged stands. The δ13C values in young stands of P. yunnanensis were not significantly different between the seasons, and those in middle-aged stands were significantly higher in summer than in spring, autumn, and winter. This result indicated that the needle δ13C values in young stands of P. armandi and middle-aged stands of P. yunnanensis were more sensitive to seasonal changes. The lower temperatures in spring and winter and the low amount of precipitation (less in spring than in winter) in this study (Fig. 2) may have contributed to the lower needle δ13C values in spring and winter in P. yunnanensis middle-aged forests, and the lower needle δ13C values in spring and higher values in winter in young P. armandi forests (Fig. 5).
Loader et al.49 suggested that higher temperatures are accompanied by an increase in plant photosynthetic capacity, resulting in higher WUE. The water status affects the water use of plants, and the transpiration rates are reduced to a greater extent than the photosynthetic rate when rainfall is insufficient. In addition, the partial closure of stomatal can increase WUE40. It is possible that the drop in temperature in winter, when the precipitation is lower than in spring, did not affect the P. armandi needle δ13C values. The plants reduced stomatal conductance when the moisture declined, resulting in a decrease in photosynthetic rate and carbon assimilation. Hence, the needle δ13C values and WUE were higher in young P. armandi forests50. In contrast, the P. yunnanensis middle-aged forests showed the same trend in winter and spring. The lower temperatures resulted in lower needle δ13C values in P. yunnanensis middle-aged forests, indicating that P. yunnanensis forests are more sensitive to temperature. This result is consistent with that of Lu et al.51, who studied WUE in oil pine. The relatively high temperature and precipitation in the summer and autumn growing season resulted in higher needle δ13C values and high water demand of P. yunnanensis and P. armandi. An increase in the temperature in the early growing season and the average growing season temperature accelerates plant transpiration and promotes plant photosynthesis. Our results suggest that the two species in the study area are affected by the combined effects of moisture and temperature, resulting in different WUE between P. yunnanensis and P. armandi during winter. The temperature had a larger effect on the WUE of P. yunnanensis middle-aged stands, whereas precipitation was the dominant factor affecting the WUE of young-aged P. armandi stands. This result is consistent with the findings of Zhou et al.52 for Acacia (Robinia pseudoacacia L.) in the Minquan, Henan, and Shanxi areas in China and Lu et al.51 for P. tabuliformis in the mountains of Beijing.
Relationships between leaf δ 13 C and meteorological factors. Meteorological factors (e.g., sunlight, atmospheric pressure, temperature, precipitation, etc.) affect plant carbon isotopes by influencing plant leaf gas exchange activities53. The dominant climatic factors are light, moisture, and temperature 2. Worldwide, WUE was highest in temperate plants (leaf δ13C value: − 2.95% to − 2.62%), followed by subtropical plants (leaf δ13C value: -3.11% to -3.05%) and tropical plants (leaf δ13C value: − 3.21% to -3.18%)2. The temperature affects the activity of enzymes involved in photosynthesis, affecting plant carbon isotope fractionation54. Some studies have shown a significant negative correlation between temperature and plant δ13C value55, whereas other studies observed a positive correlation49.
In contrast to studies showing a significant negative correlation between temperature and plant leaf δ13C values56, Yuan et al.57 concluded that the value of Pedicularis L was positively correlated with temperature. Our study found no significant correlations between the needle δ13C values of P. armandi and meteorological factors (Fig. 6). The δ13C values in middle-aged P. yunnanensis stands were significantly and positively correlated with temperature and mean surface temperature. This result indicates that the sensitivity of young and middle-aged P. armandi stands to temperature is lower than that of young P. yunnanensis stands because different species and age classes have different optimum photosynthetic temperatures.
Water also influences plant photosynthesis, respiration, and nutrient uptake and transport, as well as other biochemical processes, such as the synthesis of cell wall substances, proteins, and chlorophyll. It promotes the accumulation of sugars, proline, and other substances and affects the activity of some enzymes. If plants are water deficient, the photosynthetic capacity of the leaves decreases, and the δ13C value increases, thus increasing WUE58. Temperature and moisture are the dominant factors affecting plant photosynthesis and transpiration59. Moisture and temperature influence the atmospheric vapor pressure deficit, evapotranspiration, and mean relative humidity, affecting the leaf transpiration rate and stomatal conductance60,61. The correlation between the δ13C value and evapotranspiration, water vapor pressure, and mean relative humidity was highly significant for middle-aged P. yunnanensis forests. Only the mean air pressure was significantly negatively correlated with the δ13C value for young P. yunnanensis forests. This result indicates that temperature and moisture are the main factors affecting WUE in young and middle-aged stands of P. yunnanensis in China, and that temperature is the main factor driving the seasonal variation in δ13C of P. yunnanensis needles, with low δ13C values and low WUE at low temperatures.